The present invention is particularly applicable for filtering solids from a treated, or partially treated, waste stream prior to further treatment or discharge to a receiving stream or repository, and it will be described with particular reference thereto; however, it is appreciated that the invention has much broader applications and may be used in many installations and applications wherein effective removal of both fine, small and large particles is desired and/or required.
Waste water effluent has a large range of particles which must be removed if a sand filter of the type disclosed in prior patents is to be effectively used for filtering untreated or partially treated effluent. The particles which must be removed by the sand filter can be generally classified as discrete, gelatinous, fibrous, biotic particles and/or a variety of these combinations of these various particles having various sizes. These sizes can vary from microscopic to macroscopic. The organic portions of these many and diverse deleterious materials require oxygen and are designated as having a certain oxygen demand over a specific time, normally five days. This BOD.sub.5 remaining in the filtrate is deleterious to various water repositories into which a sand filter effluent is to be deposited. As indicated in the article incorporated by reference herein, sand filters used for waste water treatment have been effective for removal of particles greater than about two microns in size. Under this size, prior sand filters used for waste water filtering have had only slight effectiveness. This fact has seriously limited, if not completely incapacitated, prior sand filters from being used for primary treatment of waste water effluent and for treating other relatively high particle loaded effluent. Consequently, particle size below about one micron was often assumed to be a soluble solid and ignored. This led to the less than effective operation of many prior art sand filters used for tertiary treatment of waste water effluent even though published information documented large particle effectiveness. Good sand filters still allowed release of smaller particles often of the colloidal or supercolloidal size.
For some time, it has been known that the biological reaction rate in effluent increased with the reduction in the particle size of the filtered effluent. Knowing that, it has always been the objective to remove as many particles of the smallest size as can be removed from a filtered effluent. Consequently, efforts were made to remove smaller and smaller particles from the sand filtering process when treating waste effluent. This desire to remove still smaller particles has caused a large number of filter design concepts in the filtering art.
Many filtering systems have been developed and used in the filtering of treated or partially treated waste effluent with only limited success. For instance, relatively deep bed large granular particulate material has been used. The interstices in this type of bed were extremely large and permitted entry of small particles. Surface attraction of the particles to the media and physical wedging of the particles between the large granular material forming the media of the bed were both mechanical features relied upon for particle removal as effluent flowed through the bed. As more particles were lodged in the bed, the size of the interstices was reduced. Smaller flow paths increased the velocity of the effluent flowing through the bed to sweep off organic materials attracted to the surfaces of the particles. For that reason, these large particles, deep bed filters had only a limited effectiveness in the separation of organic substances from the waste effluent, but they were somewhat effective in removing discrete larger particles. Small particles were separated by surface attraction; therefore, they were ultimately stripped from the surface of the particles as the velocity of flow through the filter increased with increased loading. Consequently, the deep bed large granular material was not effective for waste water treatment.
Smaller grain media for the filter bed was used in an attempt to improve particle removal. Small interstices blocked passage of small suspended solids. Indeed, most particles were separated at the upper surface of the filter bed. As soon as that happened, the upper surface was coated and the interstices of the upper surface were blocked. This blockage occurred rapidly and limited filter runs. Indeed, often the filter run was so short that the filter could not accumulate sufficient filtered effluent for the purposes of a subsequent backwashing. This deficiency of fine grain sand filters often required additional water supplies or bypass of the sand filter itself after a short time. As can be seen, coarse grain filter beds resulted in limited small particle removal from the effluent; however, the filter runs were relatively long due to the large interstices available. To reduce the amount of particles passing through the filter, media was decreased in size. This did allow removal of more solids and thus decreased the output biological load; however, the filter runs were so short as to be unacceptable.
To solve these problems, it was suggested that several layers of media could be provided in the bed so that particles would first pass through a relatively large or coarse grain layer and then progressively move through smaller interstices at lower media levels. Such filters were not acceptable because they intermixed at the interface and packed the fine particles within the interstices of the coarse particles. This was more pronounced following backwashing which is generally performed by flowing filtered effluent in a reverse direction through the filter bed.
Effective filtration of effluent with only partial treatment, such as waste water and/or surface water, has presented a major challenge through the years for sand filters and this challenge remains today. Studies of waste water characteristics reveal that an effective filter must be able to remove a wide range of particles both of various sizes and various volumes in the effluent. These filters, although not now generally recognized or not even available, should remove colloidal and super colloidal solids that exist in waste water and are generally below a preselected size. These small particles present a significant polluting load which was ignored as a form of particles, but now can be removed by use of the improved filter and method of operation of the present invention.
Before defining the improvement in the present invention, it is recognized that many of the shortcomings of sand filters were overcome by the prior patents incorporated by reference herein as background information. This total filtering technology was of substantial commercial value and allowed fine grain sand to be used for long filter runs to effectively remove particles from waste water effluent. Even though these filters were substantial advances in the art, they still had certain disadvantages that precluded them from being used successfully for removal of smaller particles and for treatment of heavily loaded primary and secondary effluent. These prior filters, as a general technology, included a vented underdrain to develop a volume of air which was exhausted during backwashing and pulsing to extend the filter run. The air forced up through the media during filtering dislodges particles held on the surface of the sand and with a size finer than the physical passages of the interstices of the granular bed. The magnitude of this release of these fine particles is dramatically illustrated in the previously mentioned article entitled "Filtration of Primary Effluent" directed to the prior air pulsed filters. Although not generally recognized, the polluting load of the effluent was increased by release of fine particles by a factor inversely related to reduction of particle size in the outcoming filtered effluent. Consequently, these prior filters, although improvements, did somewhat encourage high velocity channeling through the sand bed, since the flow velocity through the bed is a function of porosity and gravity. As the porosity decreased with trapped particles, the flow velocity increased. Conseqeuntly, air turbulence caused by air forced from the underdrain dislodged the fine particles from the grains during upflow of air from the underdrain. This air also caused rupture of the sand in areas of least resistance to cause certain amount of problems in continued pulsing and operation in a given filtering cycle.
Prior units did not clean the lower support screen evenly during backwashing leaving crusted matter on the screen.
All of these problems and difficulties faced by sand filters as disclosed in the prior art patents are overcome by the present invention.